The present invention relates to the field of equipment for telecommunications where a frequency conversion of signals transmitted, or received is required, and more in particular to an image rejection sub-harmonic frequency converter realized in microstrip, particularly adapted to be used in mobile communication equipment.
The conversion around a desired frequency is known to be obtained through xe2x80x9cmultiplicationxe2x80x9d of a signal occupying a useful band to be converted and a sinusoidal signal of local oscillator OL at fOL frequency duly determined. The multiplication is made allowing the two signals to reach the ends of one or more non-linear devices, called mixers, such as for instance Schottky diodes, and drawing the desired conversion products from the same. From the analytical point of view, the non-linear conductance of the diodes can be developed in series of powers of values of the input current. Considering at the beginning said current as a superimposition of a pair of sinusoidal components of fOL and fRF frequencies, in the case of down conversion, or of fOLfIF frequencies, in the case of up conversion, it shall be obtained on the load, in addition to the input components, also new components generated by the diodes themselves at frequencies expressed by all possible combinations of two integer numbers m, n independent between them, mfOLxc2x1nfRF in the first case, or mfOLxc2x1nfIF in the second case, respectively. The extension to the cases in which the current of the diodes includes all the components belonging to the whole band of RF and IF signals is immediate.
As it can be noticed in both the cases, the converted signal includes, in addition to the desired useful signal (in general obtained through m=n=1), an infinite quantity of mfOL terms and of relevant conversion products of growing order that result symmetrically arranged, due to the sign xc2x1, around the mfOL frequencies.
In the case of transmission it is essential to filter the conversion products, placed outside the useful band since the same represent out-of-band spurious emissions, severely prohibited by the international regulations. In microwave applications, only one of the two side bands is generally transmitted to the double purpose of saving transmission power and reduce the band occupation. The other band, called image, being symmetrical versus the frequency mfOL, is usually suppressed like the signal of local oscillator at the frequency mfOL that is suppressed for the same reasons. It is of particular importance that one part at least of said suppression occurs before the converted signal enters the power amplification stages placed downstream the conversion stage (generally capable to equitably process both the side bands), to avoid linearity losses in the operation of the mixers and distortions caused by saturation of the power terminals.
As for reception, the signal IF at intermediate frequency is generated by the superimposition of the conversion products of the useful and image band of the RF signal. In the very frequent case in which the RF signal consists of a plurality of adjacent channels transmitted to unique side band, the conversion at intermediate frequency of the image band is an undesired effect, so that it is necessary to eliminate the image band through appropriate radiofrequency band pass filtering before the conversion, So, it is attained the purpose to avoid that the signal, or the noise present in the image band, are they too converted in the intermediate frequency band, causing superimposition or easing the worsening of the receiver characteristics.
The converters mentioned above belong to a first type, called xe2x80x9cdouble side bandxe2x80x9d and according to what said above they require a radiofrequency image filter whose realization is always complex, if one wants to maintain the useful signal unchanged, due to the closeness existing in all the embodiments of transceivers, between the two mfOL and fRF, frequencies, both much higher than the fIF.
Concerning the problem of the suppression of the components at frequency mfOL generated by the mixer, a first possible method is that to extend the rejection band of the image filter up to including again the relevant mfOL frequency components. However, should the mixer belong to a mo-demodulator of the orthogonal type (I, Q), performing the direct conversion from base band to radiofrequency, and vice versa, this approach is no more possible to be actualised since the image band does not exist and the mfOL component is at band centre of the RF signal.
A second known method to suppress the mfOL frequency components generated by the mixer, valid also for mo-demodulators, is to use two mixers, instead of one, and a balun to combine in counterphase the above mentioned components of each mixer. The main drawback of this solution is the difficult realization of a broad band operating balun, particularly at the highest frequencies; the maximum rejection that can be obtained is 30 dB approximately on 10% relative bands.
A third method, also known, to suppress the mfOL frequency components generated by the mixer, is to use a simple or double balanced structure. The two configurations are respectively obtained though a pair of diodes in antiparallel, or through a four-diode link enabling a better balancing. The rejection degree is as much higher as the physical characteristics of diodes result equal among them. Through an adequate driving of the diodes in these configurations, it is possible for instance to obtain the global transconductance including only the even order harmonics of the frequency fOL. In this case, conversion products are of the 2 mfOLxc2x1nfRF kind only, for the down converter, or 2 mfOLxc2x1nfIF for the up converter, lacking for what said above, the terms 2 mfOL in the converted signal. This means that, in the case of reception, applying a local oscillator signal OL at fOL frequency to the balanced mixer, a signal at frequency 2 mfOL+nfRF is converted at intermediate frequency fIF with the same conversion losses that would be obtained with use of a non-balanced mixer driven at the frequency 2 mfOL. Since we consider of practical interest, at least to the purposes of the invention that shall be described, only the lowest conversion order, obtained coinciding with m=n=1, we can consider the above mentioned converters as devices able to operate a conversion with local oscillator at fOL=xc2xd(fRF+fIF) frequency, or at halved frequency compared to that of local oscillators operating with the conventional converters and for this reason they are called also sub-harmonic converters. Their use involves different advantages, among which:
capability to operate at the highest frequencies with less expensive local oscillators, because as the frequency increases it is difficult to implement at low cost stable local oscillators capable to output the power required for the good operation of the mixers;
a high rejection degree of the residual components at 2fOL frequency in the converted signal, said rejection being due only to the balancing degree of the structure, that is by the coupling degree of the physical parameters of the diodes used in the mixer, rather than on the response in frequency of external networks;
a less difficult filtering of the components at the frequency fOL, considering the higher distance between fOL and the band of the useful signal at radiofrequency.
There is a second type of frequency converters differing from the type of converters mentioned up to now, due to the fact that the converters belonging to the same do not require the image filter placed at the port of the radiofrequency signal. This result is due to the adoption of a particular circuit configuration of hybrids that enables to obtain, or to use, only one of the two side bands of the RF signal. For this reasons, the converters of the second type are called xe2x80x9csingle side bandxe2x80x9d, or image rejection or suppression ones. The converters of the second type can also be implemented using sub-harmonic mixers, in this case the advantages of the converters of the first and second type are combined. Said advantages mainly derive from the use of a local oscillator frequency halved compared to that of conventional converters and at the same time, from the lack of the image filter. The scope of the present invention is an image rejection sub-harmonic converter realized in microstrip, and therefore having all the above mentioned advantages
An example of this is already known and is described in the European patent No. 322612 under the name of the same applicant, titled xe2x80x9cMicrowave image suppression harmonic frequency converter.xe2x80x9d
The mentioned converter is realized in microstrip and includes two identical harmonic balanced mixers, each one consisting of a pair of Schottky diodes connected in antiparallel, a first directional coupler in quadrature at radiofrequency, a second directional coupler in quadrature at intermediate frequency, two appropriate duplexer filters, two low-pass filters, two matching circuits and two uncoupling circuits. The first directional coupler in quadrature at radiofrequency being of the tandem type, with half-open structure and forming part of a new circuit structure whose purpose is to overcome the implementation complexity of a traditional coupler of the Lange type, used in a previous invention, in the ranges 15, 18 and 23 GHz. The structure indicated does not require a third coupler, generally used to couple the local oscillator OL signal to the mixers. The layout of the mentioned converter is particularly optimized for the ranges indicated, where the advantages of its use are considerable, however for lower use ranges, such as for instance those employed in the mobile GSM (Global System Mobile) and DCS (Digital Cellular System) systems, 900 and 1800 MHz, respectively, the Lange coupler hybrids do not show realization difficulties such to justify the invention described above. In this case the relevant layout could result too large due to the complexity of the filters and the large size the half-open structure tandem coupler would have.
Object of the present invention is to overcome the above mentioned drawbacks and to indicate an image rejection sub-harmonic frequency converter whose layout in microstrip results of small size and simple to realize in the frequency ranges used in the mobile communication GSM and DCS, advantages obtained through a new combination of means employing traditional hybrids rather than inventive hybrids as in the mentioned background art, and filtering structures reduced to simple stubs or to filters made at concentrated parameters. For description sake, but without departing from the scope of the invention, reference is made hereinafter only to the circuit employing stubs in xcex/4.
The above object is solved by the combination of features of the claim 1 regarding an image rejection sub-harmonic frequency converter, microstrip realized, particularly for use in mobile communication systems, characterized in that it includes:
a) a first double side band frequency sub-harmonic mixer consisting of antiparallel diodes placed between a first and a second filtering structure connected to relevant microstrips connecting at one end of said diodes a first port, reached by a local oscillator signal OL, and at the other end a second port for a radiofrequency signal RF; said mixer including also a third port for a signal at intermediate frequency IF, connected to one end of said diodes in antiparallel through a low-pass filter that allows said signal at intermediate frequency IF to pass and blocking said OL and RF signals;
b) a second frequency mixer identical to said first mixer;
c) a first 3 dB, 0xc2x0 directional coupling circuit at local oscillator frequency, coupling said OL signal to said first ports of said frequency mixers;
d) a second 3 dB, 90xc2x0 directional coupling circuit at radiofrequency, Lange coupler type, or equivalent, coupling said signal RF to said second ports of said frequency mixers;
e) a third 3 dB, 90xc2x0 directional coupling circuit at intermediate frequency coupling said signal IF to said third ports of said frequency mixers.
In some known applications, it was noticed that the structure of directional couplers, called also hybrids, differ from that of the present invention due to the fact that the hybrid at local oscillator frequency is 3 dB, 45xc2x0 and the radiofrequency hybrid is at 3 dB, 0xc2x0; in such a way the total phase shifting at 2 fol frequency is 90xc2x0, as it must be to define the correct phase relation enabling to sum in counterphase the image band. This solution shows a narrower rejection band versus the structure of hybrids according to the invention since, while the realization of broad band 3 dB, 0xc2x0 and 3 dB, 90 is rather simple, that of a 3 dB, 45 hybrid is not so simple. Among those application there is the document DE-A-2 608 939, which appears to be the closest prior art. In the cited application a stripline mixer both sub-harmonic and image rejection is disclosed. The mixer uses two 3 dB couplers, or hybrid rings, for coupling IF and OL signals at two respective ports of two symmetric couple of diodes in antiparallel acting as a balanced mixer, while RF signal is fed directly to the diodes. A delay line L dimensioned in a way to introduce 45xc2x0 phase displacement at the OL frequency is inserted between one output of the OL hybrid and the OL port of a couple of diodes. Each couple of antiparallel diodes is connected at one end to a short-circuited quarter wavelength line, at the OL sub-harmonic frequency, and at the other end to an open line of the same electrical length. The outlined drawbacks don""t entirely apply to the mixer of D1, but the introduction of the delay line L, due to the particular combination of hybrid circuits, is an additional complication which doesn""t afflict-our mixer.
In the mobile communication field, the converter structure described in the claim 1 enables to obtain the following system advantages, compared to the present use of the traditional converters:
minimum occupation of the substrate at use frequencies, if the frequency is high, it is convenient to realize the structure with stubs, otherwise it is convenient for space requirements, to realize the filtering structures through concentrated parameters L and C;
absence, or about, of residual interferingt components at the frequency 2 fOL in the radiofrequency signal RF, and easy filtering of the local oscillator component at fOL;
best rejection of the image band and easy filtering of a possible residue;
when 4 diodes are used in antiparallel configuration by pairs, the linearity of the device implemented improves.
higher accuracy in the execution of the frequency hopping: it is useful to point out that the frequency hopping is a planned frequency jump of the carrier assigned to a generic radio channel, made at each time slot, to prevent on a statistical basis the dangerous effects due to quick fadings of the radio signal caused by multiple paths (Rayleigh fading), or by other interference causes. The higher accuracy derives from the fact that, compared to a traditional converter, it is necessary one half of the total phase variation, at equal xcex94t, to obtain the same frequency jump xcex94f. This involves the possibility to use a higher linearity area of the voltage/frequency transfer feature of the voltage controlled oscillator, or VCO, physically implementing the frequency hopping or,. alternatively, the possibility to use VCOs having less stringent characteristics of linearity, tuning interval and bandwidth.
simplification in the introduction of the operation method called xe2x80x9cDual Modexe2x80x9d through which a same terminal equipment can switch its operation from a GSM 900 MHz type network to one of the DCS 1800 MHz type. In fact, it is possible to use one sole local oscillator at frequency (900+fFI) MHz to supply two converters, out of which the first of the traditional type (the one for the GSM), and the second one of the sub-harmonic type (that for DCS 1800 MHz).
Concerning this last advantage, it is possible to configure another object of the invention consisting of a mobile terminal equipment with the possibility to switch its own operation from a first mobile system to a second mobile system operating in an higher area of the radiofrequency spectrum, and vice versa, characterized in that it includes one sole local oscillator and switching means of the local oscillator signal towards a traditional frequency converter included in a radio chain belonging to said first mobile system, or towards an image rejection sub-harmonic frequency converter of the claim 1 included in a radio chain belonging to said second mobile system, as described in claim 6.
A mobile terminal equipment with switching band capability is disclosed in the European patent application EP-A-0 581 573. The invention concerns an universal radio telephone whose circuital arrangement includes three independent PLL circuits: two first for generating UHF local oscillator signals for driving the reception and transmitting frequency converters, and a third VHF one for driving a digital (I, Q) modulator/demodulator. The VCOs embedded in the two UHF PLLs drive respective multiplier means (11, 12) electronically controlled for either selecting the fundamental or the second harmonic of the driving signals. The UHF signals selected by multplier means (11, 12) fed the local oscillator input of up and down conversion mixers respectively. Only one intermediate frequency IF is used in the reception branch.
The approach to effect xe2x80x9cdual modexe2x80x9d in the cited invention consists of keeping unchanged the mixer operating mode and change the frequency of the driving OL signal, in case, by the selection of the second harmonic generated from the output of the OL oscillator. Lacking a description of the multiplier means (11, 12), the selection of the second harmonic is supposed obtained by filtering out a second harmonic generated from a non-linear device the UHF signal goes through. This approach, omitting for sake of simplicity the relevant shortcomings, is quite different from our method to implement xe2x80x9cdual modexe2x80x9d in which the local oscillator frequency is kept constant and the mixer used in the radio chain is switched from a traditional to a sub-harmonic one.
From the description of the invention given up to now it can be noticed that it is sufficient to suppress the 3 dB, 90xc2x0 directional coupler at intermediate frequency, replace the 3 dB, 90xc2x0 directional coupler at radiofrequency with a 3 dB, 0xc2x0 one, and replace in each mixer the low-pass filter allowing signal at intermediate frequency IF to pass by a low-pass filter coupled to the transmission pulse, because the remaining structure of the converter be suitable to be used also as sub-harmonic mo-demodulator of the orthogonal type (I, Q), able to convert the signal in base band directly to radiofrequency, and vice versa.
Therefore a further object of the invention is an orthogonal sub-harmonic mo-demodulator, as described in claim 7, which could find application in future implementations of mobile systems.